1. Field of the Invention
This invention relates to an imaging system and an imaging method for imaging a sample in a medium carried in a container as an imaging object.
2. Description of the Related Art
In medical and bio-scientific experiments, liquid or gel-like fluid (e.g. culture solution, medium or the like) is poured into each well of a plate-like tool (e.g. called microplate, microtiter plate or the like) in which a multitude of recesses also called as wells are, for example, arrayed, and things cultured herein such as cells are observed and measured as samples. In recent years, samples have been imaged and digitized by a CCD camera or the like and various image analysis techniques have been applied to image data for observation and analysis.
In an imaging system of this type, a correlation between actual optical densities of imaging objects and gradation values obtained by expressing the optical densities by multi-gradation image data is not necessarily linear due to nonlinearity in the sensitivity of an, imaging system. Thus, a gradation correction (also called as a gamma correction or a tone correction) to properly adjust this is necessary. A technique applied to a document scanner for optically reading a plane document is known as a precedent of such a gradation correction technique. For example, in an image processing apparatus described in JP2003-209693A, pre-scanning is performed with a low resolution prior to actual scanning in scanning and reading a document image. A correction characteristic for a gradation correction, specifically a correlation between input gradation values and output gradation values is obtained from a histogram distribution characteristic of an obtained image and a gradation correction processing is performed during actual scanning.
It is considered to apply a gradation correction technique like the conventional technique also to an imaging system used for the purpose of observing samples such as cells. If imaging objects are samples in a culture medium, the medium is not perfectly transparent and has an optical density of a certain degree. Thus, there is a characteristic that a luminance of each pixel of an imaged image is not higher than a luminance corresponding to the medium itself. However, such a problem is naturally not considered in the above conventional technique aiming to read a document and only a correlation between luminance values and gradation values at intermediate gradations is focused. Thus, if this conventional technique is directly applied to the imaging of biological samples, gradation values are assigned up to a luminance level which is not applicable in an actual image, wherefore there has been a problem that a range of invalid gradation values not used in the multi-gradation expression of an image is created and a dynamic range of density expression in a multi-gradation image data is limited.